(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of reducing dopant-out diffusion occurring in channel regions of PMOS/NMOS gate electrodes having a channel width of 0.25 xcexcm or less.
(2) Description of the Prior Art
With the continued advancement of the semiconductor technology and the fabrication of Integrated Circuit (IC) devices, the components and component features that are part of these devices continue to decrease in dimension. Semiconductor devices can essentially be broken down into bipolar devices and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices, whereby the latter category forms an increasing percentage of the total number of devices that are used in Integrated Circuit (IC) applications. It is projected that MOSFET devices will constitute roughly 90% of the overall market whereas the bipolar devices will be used for the remaining 10% of the applications. With reductions in device size is required a reduction in device power consumption, which imposes the requirement of decreased device feature lengths. It can be stated as a general rule that device speed varies inversely proportional with device feature length while power consumption increases approximately with the square of the device feature length. Feature size currently being approached is in the micron and sub-micron or 0.5 xcexcm range where it is not considered impossible that the feature size of 0.2 xcexcm will become a reality in the near future.
Field Effect Transistors (FET) are at this time used extensively in Ultra Large Scale Integration (ULSI) applications. FET are formed using gate electrodes, usually made of polysilicon, and adjacent source/drain regions to which self-aligned source/drain contact areas are established. In its basic form, a Metal Oxide Semiconductor (MOS) transistor has a gate electrode to which a voltage is applied. The gate is created on the surface of a silicon substrate; the voltage that is applied to the gate creates an electric field that is perpendicular to the interface between the gate electrode and the substrate. The areas in the substrate immediately adjacent to the gate electrode are doped, thereby varying their electric conductivity. These areas become the source and drain regions. By varying the voltage that is applied to the gate electrode, the electric field in the interface between the gate and the substrate interface can be varied and, with that, the current that flows between the source and the drain regions. This electric field controls the flow of current through the device, the device is therefore referred to as the Field Effect Transistor.
The type of device that is created and the type of areas that are created in conjunction with the device are to a large extent determined by the type of dopant that is used and the processing conditions under which the dopants are applied. The creation of semiconductor devices starts with a substrate, which is any material that can retain dopant ions, and the isolated conductivity regions brought about by those ions. Typically, a substrate is a silicon-based material, which receives p-type or n-type ions. The device features that are created dictate the type of doping and doping conditions. For instance, boron or phosphorous can be used as a dopant and can be doped into polysilicon layers or into polycide gate electrodes.
Channel stop dopants can be p-type or n-type; implants can contain a p-type dopant such as boron implanted at a dose of 5xc3x971013 atoms/cm2 at an energy of 35 keV. An n-type dopant is P31 implanted at a dose in the order of 2.8xc3x971012 atoms/cm2 at energy of 60 keV.
A typical conductivity imparting dopant, used to create a lightly doped source and drain region, is phosphorous, ion implanted at an energy between about 5 to 100 KeV, at a dose between about 1E11 to 1E14 atoms/cm2. A medium doped source and drain region can be created by using arsenic or phosphorous, ion implanted at an energy between about 5 to 50 KeV, at a dose between about 1E12 to 5E14 atoms/cm2. A heavily doped source and drain region can be created by using arsenic, ion implanted at an energy between about 5 to 150 KeV, at a dose between about 1E15 to 1E16 atoms/cm2.
Dual gate transistor design is the design where both NMOS and PMOS devices are created on the same chip. Earlier designs of Metal Oxide Semiconductor (MOS) devices primarily used PMOS devices. In its early history, the CMOS transistor was considered to be only an extension of the design for MOS IC""s.
Later advancements in fabrication technology, mostly due to improvements in ion implant techniques, allowed for the PMOS devices to be replaced with NMOS devices. The larger drive current of NMOS devices resulted in faster speed of these devices, which resulted in NMOS devices becoming the dominant device type in the IC industry.
NMOS devices however exhibited severe limitations in power density and power dissipation, causing CMOS devices to become the dominant technology for IC device manufacturing. With the arrival of CMOS devices, a renewed interest in PMOS devices developed.
CMOS employs both NMOS and PMOS devices to form logic elements. The advantage of CMOS is that its logic devices draw significant current only during the transition from one logic state to the other while drawing very little current between this transition.
The scaling of the CMOS devices in the sub-micrometer device range presents a major challenge. For the fabrication of p-channel and n-channel devices, n+doped polysilicon gates are used resulting in functional asymmetry. A number of techniques have been used to assure that the p-channel and n-channel devices are completely symmetrical in their performance characteristics such as threshold voltages, device dimensions and doping while the p-channel device is, for ease of manufacturing, a surface channel device. These devices are made using undoped polysilicon for the gate structures that are simultaneously doped at the time that the source/drain regions of each type of device are implanted. This leads to special manufacturing problems caused by, among others, diffusion of impurity implants through the gate oxide into the channel region thereby changing the threshold voltage of the device. Another concern in creating dual-gate CMOS devices is that various dopants may inter-diffuse between adjacent regions, an effect that can become critical at high anneal and other processing temperatures.
Increased CMOS device speed however requires short channel length, the design of p-channel devices with short channel length presents unique problems mostly centered on methods of doping and pocket implants for the device and the impact that these methods have on PMOS device characteristics. A technique used for instance to create deeper and narrower implants is to increase implant energy and implant dosage. This approach however may negate the self-alignment aspect of the implants where the gate electrode serves as a shield and the implants become in this way aligned around the gate electrode. The high implant energy and dosage may result in implant penetration through the gate electrode thereby affecting the gate threshold voltage performance while the high implant energy and dosage may affect the thin layer of gate oxide underlying the gate electrode. It is therefore critical to design an implant method and sequence where gate penetration by implant dopants is not a factor.
Various types of implants are used in the industry to create semiconductor devices. Implants can be a well implant that provides a more uniform background doping. A punch-through implant provides a channel with greater robustness to punch-through voltage. A thresh-hold implant sets the thresh-hold voltage of a device (like an IGFET). The well implant can be provided by boron at a dose in the range of 1xc3x971012 to 1xc3x971013 atoms/cm2 and an energy in the range of 100 to 170 kilo-electron volts, a punch-through implant can be provided by boron at an dose in the range of 1xc3x97102 to 1xc3x971013 atoms/cm2 and an energy in the range of 40 to 100 kilo electron volts, the thresh-hold implant can be provided by boron at a dose in the range 1xc3x971012 to 1xc3x971013 atoms/cm2 and an energy in the range of 2 to 30 kilo electron volts. A channel implant can have a boron concentration on the order of 1xc3x971017 atoms/cm2. Implants can also use arsenic; this can form an n-doped region. A heavy doped implant for instance is 3-5xc3x971015/cm2 of arsenic at 50-80 keV.
FIG. 1a gives an overview of the self-aligned source, drain and gate salicide formation. This process starts with the surface of a semiconductor substrate 10xe2x80x2, FIG. 1a. Forming insulation regions 12xe2x80x2 that bound the active region isolates the active region that is to be used for the creation of, for instance, a gate electrode. Field Oxide (FOX) isolation regions 12xe2x80x2 can be used to electrically isolate the discrete devices, such as Field Effect Transistors (FET""s) in ULSI circuits on semiconductor chips formed from silicon substrate. One conventional approach in the semiconductor industry for forming field isolation is by the Local Oxidation of Silicon (LOCOS) method. LOCOS uses a patterned silicon nitride (Si3N4) as an oxidation barrier mask, the silicon substrate is selectively oxidized to form the semi-planar isolation. However, this method requires long oxidation times (thermal budget) and lateral oxidation under the barrier mask limits the minimum spacing between adjacent active device areas, and therefore prevents further increase in device packaging density.
One method of circumventing the LOCOS limitations and to further reduce the field oxide (FOX) minimum features size is to allow shallow trench isolation (STI). One method of making STI is to first etch trenches having essentially vertical sidewalls in the silicon substrate. The trenches are then filled with a CVD of silicon oxide (SiO2) and the SiO2 is then plasma etched back or polished back using CMP, to form the STI isolation region. These regions are indicated as regions 12xe2x80x2 in FIG. 1a. 
A thin layer 16xe2x80x2 of gate oxide is thermally grown over the surface of the substrate 10xe2x80x2 in the active device region. To create the gate structure, a layer 14xe2x80x2 of polysilicon is grown over the thin layer 16xe2x80x2 of gate oxide. The polysilicon layer 14xe2x80x2 is masked and the exposed polysilicon and the thin layer of oxide are etched to create the polysilicon gate 14xe2x80x2 that is separated from the substrate by the remaining thin layer of oxide 16xe2x80x2. The doping of the source/drain regions starts with creating the lightly N+ doped diffusion (LDD) regions 32xe2x80x2/34xe2x80x2. The sidewall spacers 22xe2x80x2 for the gate structure are formed after which the source and drain region doping is completed by doping the source/drain regions 18xe2x80x2/20xe2x80x2 to the desired level of conductivity using a N+ dopant.
Contact points to the source/drain regions and the electrode gate are then formed by first selectively depositing a layer of titanium over the surface of the source/drain regions and the top surface of the gate electrode. This titanium is annealed causing the deposited titanium to react with the underlying silicon of the source/gain regions and the doped surface of the gate electrode. This anneal forms layers of titanium silicide 24xe2x80x2/26xe2x80x2 on the surfaces of the source/drain regions and layer 28xe2x80x2 on the top surface of the gate electrode.
The metal contacts with the source/drain regions and the gate electrode are formed as a final step. A dielectric 30xe2x80x2 such as silicon oxide is blanket deposited over the surface of the created structure, patterned and etched to create contact openings 36xe2x80x2/37xe2x80x2 over the source/drain regions and opening 38xe2x80x2 over the top surface of the gate electrode. The metalization layer selectively deposited over the patterned dielectric establishes the electrical contacts 40xe2x80x2/42xe2x80x2 with the source/drain regions and 44xe2x80x2 with the top surface of the gate electrode.
One of the key factors that affects the reliability of FET devices of small geometry results from the shrinkage of the channel length and channel width. To overcome problems associate with short and narrow channel effects is therefore an important concern in the design of FET devices. Gate oxide integrity can also be negatively affected by the hot-carrier effect. If the carriers can acquire sufficient energy from the lateral electric field (the field parallel to the plane of the substrate surface), these carriers may transfer across the substrate to the gate oxide interface thereby affecting the oxide conduction band and, ultimately, its function of forming a gate oxide layer of electrical separation. The electric field barrier for electron injection is smaller than it is for hole injection. This problem is therefore more prominent in n-channel MOSFET""s because electrons form the charge carrier in the device channel.
For device features below the 0.5 xcexcm range (deep submicron), thinner polysilicon is required for the gate electrode. A relatively thick layer of polysilicon when used for the electrode gate structure results in poly depletion and a larger effective time required for the oxidation process of the gate electrode, which results in lower drain saturation current and a higher threshold voltage for the gate electrode. By limiting the thickness of the layer of polysilicon, the energy that can be used to perform the pocket implants must be reduced, resulting in shallow implants.
Because of these processing issues, the application of the various implants requires considerable process development and in some instances new processes have to be used. Using conventional processing techniques to create deep and well-defined pocket implants, the implant energy and implant dosage cannot be increased. A high-energy pocket implant may result in penetration of the implant through the polysilicon of the gate electrode thereby affecting the threshold voltage of the device. High dopant concentration will degrade the quality of the thin layer of gate oxide underlying the gate polysilicon.
For the creation of gate electrodes that have a channel length of 0.25 xcexcm or less two aspects require special considerations. These two aspects are the reverse narrow width effect and the short channel effect. The issue of short channel effect has been discussed above. The invention addresses the reverse narrow width effect of creating sub-micron gate electrodes. This effect is increasingly important for the creation of for instance low power Static Random Access Memory (SRAM) devices since these devices are required to have good narrow channel width performance.
The reverse channel width effect is the occurrence of channel dopant diffusing from the channel region out to the STI oxide region causing rounding of the STI trench. The result is that the dopant concentration at the edges of the channel region that interfaces with the STI regions is reduced with respect to the center of the channel region of the gate electrode. A number of innovations have been provided, which are aimed at preventing this channel dopant out-diffusion, such as providing a silicon nitride liner in the STI trenches or providing a layer of nitride around the upper edge of the STI trenches. Another method has been provided whereby the loss of channel dopant is compensated by providing additional channel doping. However, increasing the dopant that is provided for the channel region most frequently results in increased out diffusion of the dopant to the surrounding STI regions while increased dopant concentration in the channel region further degrades the interface junction between the silicon substrate and the body of the gate electrode. The invention provides a method that compensates for the loss of dopant concentration around the perimeter of the channel region where this channel region interfaces with the surrounding STI region.
U.S. Pat. No. 6,121,096 (Hopper) shows an angle implant into a channel. However, this reference differs from the invention.
U.S. Pat. No. 5,240,874 (Roberts) and U.S. Pat. No. 6,083,795 (Liang et al.) show angle implant into corners to improve reverse narrow width effect.
A principle objective of the invention is to eliminate the negative impact on the channel region and the surrounding Shallow Trench Isolation regions of Field Effect Transistor (FET) devices that is incurred by out-diffusion of dopants from the channel region to surrounding Shallow Trench Isolation regions.
Another objective of the invention is to eliminate undesirable dopant distribution in the channel region of Field Effect Transistors caused by out-diffusion of dopant from the channel region of the Field Effect Transistors.
Yet another objective of the invention is to eliminate corner rounding or undesirable impurity concentrations in the Shallow Trench Isolation (STI) regions of Field Effect Transistors due to dopant concentrations in these STI regions incurred as a result of out-diffusion of dopant from the channel region.
In accordance with the objectives of the invention a new angle implant is provided that reduces or eliminates the effects of narrow channel impurity diffusion to surrounding regions of insulation. A layer of pad oxide is created over the surface of a silicon substrate, a layer of silicon nitride is deposited and patterned such that the layer of pad oxide is exposed where Shallow Trench Isolation regions are to be created. A layer of photoresist is deposited, patterned and etched to expose the surface of the p-well that has been created in the surface of the substrate, p-type impurity is then implanted into the corners of the STI region that are adjacent to NMOS device that is to be created over the p-well. The process is then repeated in reverse image order to perform a n-type implant into the corners of the STI region that are adjacent to the PMOS device that is to be created over a n-well region that has been created in the surface of the substrate. The p-type and n-type implants are angle implants that penetrate under the patterned layer of silicon nitride, thus penetrating into the corners of the STI regions underlying the patterned layers of silicon nitride. The substrate is, after the p-type and n-type angle implants, processed in the conventional manner to create STI trenches, fill the trenches with oxide and planarize the surface of the oxide that has been deposited inside the STI trenches. The formation of the N-well and the p-well use the same mask as the mask that is used for the angle implant.